These studies will be concerned with investigations into the molecular mechanisms of DNA repair in bacteriophage and mammalian cells and the relationship of DNA repair to carcinogenesis. A phage-induced UV specific endonuclease will be used as a prove for pyrimidine dimers in DNA both in vitro in vivo. In addition, a recently identified phage- induced dimer excision nuclease will be purified and characterized. These two enzymes will be used in a study in a study aimed at carrying out complete DNA repair in vitro. Attempts will be made to identify and purify both a repair specific endonuclease and dimer excision nuclease in extracts of mammalian cells and enzymatic comparisons will be made between normal and xeroderma pigmentosum cells from humans. Attempts will be made to promote chemical carcinogenesis of human diploid cells in culture by specific manipulation of the DNA repair system. The effect of DNA repair inhibition on malignant transformation by chemicals of rodent cell lines in cultures will also be examined.